In conventional wireless devices such as radiotelephones, it is often necessary to generate a local frequency for use as a fundamental frequency reference for the radiotelephone. One example of the use of local frequencies is seen in a dual mode radiotelephone, such as a radiotelephone which operates in the 800 MHz frequency range for cellular services and in the 1900 MHz range for personal communication services (PCS). In such a radiotelephone, the cellular and PCS bands are divided into a plurality of narrow bandwidth channels, such as 30 KHz wide channels for D-AMPS, 200 KHz channels for GSM or 1.8 MHz channels for CDMA. Such a radiotelephone typically generates a local frequency to be used by transceivers in the radiotelephone to modulate and demodulate signals in these frequency ranges. Accordingly, the frequency accuracy of the radiotelephone should be smaller than the channel spacing. Furthermore, for digital transmission, the frequency accuracy typically must be a small fraction of the information transmission rate, irrespective of the width of the channel. For example, even though an IS95 device may operate with 1.8 MHz channel spacing, a frequency accuracy of 100 Hz may be required.
To obtain these high accuracies, for example, 100 Hz at a carrier frequency of 2 GHz, an accuracy of 5 parts in 10 to the power of 8 may be necessary. Such an accuracy is greater than typically can be achieved by an open loop crystal oscillator. Thus, closed loop synchronization is typically utilized to increase frequency accuracy.
A closed loop system is typically implemented using a voltage controlled oscillator. The local frequency is generated by the voltage controlled oscillator and is synchronized to a frequency transmitted by a base station communicating with the wireless device. Thus, for example, in a conventional radiotelephone, a cellular base station transmits a synchronizing frequency which is received by the radiotelephone and compared to the locally generated frequency. The error between the synchronizing signal and the generated frequency is then typically used to adjust the control voltage to the voltage controlled oscillator to reduce the error between the two signals.
One problem with conventional voltage controlled oscillators is that, typically, the relationship between voltage and frequency may vary with temperature. Furthermore, the variation in frequency may change over time, thereby reducing the likelihood that a single temperature compensation would be suitable for the operational lifetime of the radiotelephone.
A further problem may arise from inaccuracies in the frequencies transmitted by base stations. Older base stations may provide inaccurate frequencies or the radiotelephone may have such a great initial error that a base station signal cannot be detected. Thus, it may not be practical to utilize synchronization with a base station at a given temperature to determine a temperature compensation value for a voltage controlled oscillator because the error in the signal may reflect variance in the synchronizing signal rather than the temperature variation in the voltage controlled oscillator. Conventional radiotelephones might have used averaging of compensation values across multiple base stations so as to reduce the impact of a few inaccurate base stations. The radiotelephone would then learn over time the temperature variations of the voltage controlled oscillator to compensate for such variations. Conventional radiotelephones can also, however, suffer temperature variations in the control sensitivity of their voltage controlled reference oscillators, which were typically not compensated in the prior art.
A further difficulty which may arise in dual band radiotelephones may result from the different operating frequencies of the radiotelephone. The damping factor and gain of the feedback control of the synchronizer circuit of the radiotelephone affect the minimal frequency error or the rate at which the voltage controlled oscillator reaches the synchronized frequency. As a result of the voltage controlled oscillator switching between two frequencies, the gain or damping factor of the feedback in the synchronization circuit of the radiotelephone may change thus, possibly, resulting in less than optimal performance for one or both of the frequencies.